This was the last of 3 different types of hand warmers i have tried and in my opinion it provides the best value for money and does the best job.
I have tried the Charcoal hand warmers which burn a stick of charcoal they tend to last about 4-5 hours at best and they obviously have a very strong burning smell which is not ideal if you work around other people etc, they do get very hot maybe too hot at times.
next i tried the small Sodium Acetate hand warmers which is contained in a small plastic pouch which you snap a small metal disc and a chemical reaction generates heat, now these don't get much more than luke warm and last for less than an hour, and if you don't recharge them correctly they become useless.
then i purchased one of these Zippo hand warmers, i had high hopes from the start purely based on the reputation they have for their lighters, and they have not let the quality slip with the hand warmer, you can pick up 250ml of fuel for £3-£4 which should last you a good 2-3 weeks depending on how often you use the hand warmer, the warmer itself can get too hot to touch but with it in its pouch it is the ideal temperature, its easy to refill using the measuring jug that is provided, you simply remove the burner filament and pour the fuel in using the supplied measuring jug, now this listing says it last up to 12 hours, mine today lasted over 19 hours i lit it at 4:30 am before i left for work and it was still well alight at 2330 that evening, it burns with no flame virtually odourless just the usual smell of Zippo fluid, to stop the hand warmer simply use a tool the remove the hot filament and it will cool within 2-3 minutes.
Again in my opinion out of the 3 types i have tried this is by far the best i've used and i have no doubt it will last for years to come.